Optical receivers for light guide systems generally use a high frequency photodiode to convert the light signal to a photocurrent, which has both an a.c. (alternating current) and a d.c. (direct current) component. It is desirable to amplify the photocurrent signal to a higher voltage as early as possible in order to make it relatively insensitive to the introduction of ambient noise. To this end, optical receivers include a so-called "front end" transimpedance amplifier which raises the voltage level of the signal several orders of magnitude. The output of the front end is then further amplified and shaped in a linear channel section.
It is desirable for an optical receiver to have a wide dynamic range, so that it can be used for assorted cable lengths; each cable length will have a different light attenuation, and therefore a different light intensity output. A limiting factor in the dynamic range is the dynamic range of the transimpedance amplifier. Excessive a.c. or d.c. input will cause the amplifier to saturate.
The excess a.c. component of the signal can be shunted away by means of a shunt transistor which is controlled by a controller responsive to the output of the transimpedance amplifier. Such an arrangement is described in U.S. Pat. No. 4,415,803 issued to Tran V. Muoi Nov. 15, 1983 and assigned to the present assignee. When the circuit is implemented in FET (field effect transistor) technology, however, the shunting action of the shunt impedance has the effect of disturbing the bias voltage at the input port of the transimpedance amplifier. For an FET amplifier, it is important that this voltage be maintained accurately at an appropriate bias level. Otherwise, the input transistors of the amplifier will not be operating in their proper gain mode.
One way of maintaining the proper bias at the input of the amplifier is by means of a current mirror circuit, as is described in the copending application Ser. No. 401,521 now U.S. Pat. No. 4,498,001 entitled "Transimpedance Amplifier For Optical Receivers," which is also assigned to the present assignee. It would be desirable to have an alternative means of maintaining the proper bias which requires less complexity.